The present invention relates to a method for wrapping filter bags for products for infusion such as coffee, tea, camomile and the like in sealed outer envelopes and the machine that implements the method.
At present, machines for making filter bags comprise a plurality of stations, located one after the other, in which the filter bags are made. Each filter bag contains a charge of product closed in a corresponding chamber. Depending on the type of product it contains and the machine used to make it, the filter bag has one or two chambers and is joined in one of several different known ways (for example, by knotting, heat sealing or by means of a heat sealed stamp) to a string that connects it to a pick-up tag which is joined in one of several different known ways (for example, by knotting, heat sealing or by means of a stamp) to the free end of the string.
The string and the pick-up tag are suitably positioned on the filter bag thus made (for example wound around the bag and connected to it or tucked inside it) to form a product that as ready to be packed in suitable cartons or boxes.
Before groups of filter bags are packed in boxes, the filter bags may be individually wrapped in an outer envelope consisting of a sheet of wrapping material folded around each filter bag and heat sealed in one or two different ways: on at least two longitudinal sides or edges of the outer envelope if the latter has an opening flap folded over one of the surfaces of the envelope itself; or simultaneous heat sealing on three sides of the outer envelope, that is to say, two longitudinal edges and one transversal edge, if the envelope is of the simple type, that is to say, without the opening flap.
Known machines used to wrap the filter bags in individual outer envelopes have a filter bag handling station comprising a plurality of grippers mounted radially on a starwheel, each being designed to pick up a single filter bag. As the starwheel turns, the filter bags are carried to different stations where the string and tag are joined to each other and to the filter bag and then to an wrapping station where a web of wrapping material is cut into separate sheets, each of which will form the outer envelope of a corresponding filter bag.
Close to this station, the filter bag is moved radially away from the starwheel to enable the sheet of wrapping material to be fed between it and the starwheel. The filter bag is then moved close to the starwheel again to enable the sheet of wrapping material to be folded into a U shape around the filter bag. During this step, the opening flap of the outer envelope may also be formed by suitable folding means at the station, the flap being held in place by a presser element on the gripper elements on the starwheel.
After this, the filter bag with the sheet of wrapping material folded in a U shape around it, is carried to a sealing station where two or three edges of the sheet formed by the previous folding operation are sealed by suitable sealing means consisting of two or three elements depending on the type of outer envelope to be made.
The filter bag thus sealed in its outer envelope is then fed to the stacking and final cartoning stations.
This method, consisting in sealing the edges of the outer envelope after simply folding it, often results in a poor quality finish owing to the difficulty of correctly sealing the two longitudinal edges and, in the case of a simple outer envelope without opening flap, the transversal edge simultaneously. Indeed, during the folding step, the longitudinal edges in particular, are difficult to place squarely over each other since the wrapping material follows the shape of the filter bag which is not firm enough to enable a neat, square fold to be made. This results in skew seals and crinkling along the edges of the outer envelope which are not only unattractive but which also do not adequately protect the product inside. This problem is accentuated by the physical properties of the wrapping material from which the outer envelope is made and whose relative rigidity prevents it from keeping its shape before and after sealing.
The present invention has for an object to overcome the above mentioned problem by providing a method and a machine for wrapping filter bags in sealed outer envelopes in an extremely simple, fast and efficient manner without altering the general structure or existing machinery for this purpose, so as to make outer envelopes of good quality and ensuring a proper seal.
Accordingly, the invention provides a method for wrapping filter bags for products for infusion in individual sealed outer envelopes, the filter bags being of the type comprising at least one chamber containing a charge of product and a string connected at one end to the chamber itself, while the other end of the string is connected to a pick-up tag, the method comprising the following steps: feeding a succession of single, plane sheets to a station or wrapping each sheet over a corresponding filter bag; folding the sheet into a re-shape about a transversal axis of the sheet itself so as to form at least two opposite wings between which the filter bag is placed; and joining at least the longitudinal edges of the U-shaped sheet to form a sealed outer envelope around the filter bag; the method being characterised in that between the step of folding the sheet and the step of joining its longitudinal edges, it comprises a further step of rolling the longitudinal edges so as to flatten them.
The present invention also provides a machine for wrapping filter bags for products for infusion in individual sealed envelopes, the falter bags being of the type comprising at least one chamber containing a charge of product and a string connected at one end to the chamber itself, while the other end of the string is connected to a pick-up tag, the machine comprising handling means for moving each filter bag in a working direction towards a station for wrapping a single, plane sheet of wrapping material around the filter bag; means for folding the sheet of wrapping material about a transversal axis of it to form a U-shaped wrapping around the filter bag, with at least two opposite wings between which the filter bag is placed; and a station for joining at least two longitudinal edges of the U-shaped wrapping sheet to form a single sealed envelope around the filter bag; the machine further comprising between the wrapping station and the joining station a rolling station designed to flatten the longitudinal edges.